1. Field of the Invention
The present invention relates to a laser beam optical scanning system and to a laser printer provided with the laser beam optical scanning system.
2. Description of Related Art
In a laser beam optical scanning system which is installed in an electrophotographic laser printer to write an image on a photosensitive drum, a laser diode has been used as a light source. Since a laser beam emitted from the laser diode is a divergent pencil of rays, a convergent lens (collimator lens) is disposed in front of the laser diode so as to change the divergent pencil of rays into a parallel or a convergent pencil of rays.
A Fresnel lens has been developed. The Fresnel lens has a sawtooth surface and has a pattern of concentric circles which are pitched by microns and zones among which are alternately transparent and opaque. The Fresnel lens uses refraction and diffraction and bends incident rays at the alternately transparent and opaque zones. The Fresnel lens changes a parallel pencil of rays into a convergent pencil of rays and changes a divergent pencil of rays emitted from a focal point of the Fresnel lens into a parallel pencil of rays.
It is possible to install the Fresnel lens in a laser source unit instead of the collimator lens. In this case, however, there is a problem that a change in the optical performance (defocus) is caused by a change in the wavelength of the laser beam emitted from the laser diode. The wavelength of the laser beam emitted from the laser diode changes when the temperature of the laser diode changes. Additionally, a rise in the temperature of the circumstances lowers the emission output of the laser diode, and in order to make up for the fall in the emission output of the laser diode, the laser diode must be supplied with a larger driving current. The supply of a larger current increases the heat value of the laser diode, and consequently the laser diode emits a laser beam with a longer wavelength. Since the Fresnel lens uses diffraction, the Fresnel lens changes its focal length with a change in the wavelength of the laser beam. The change in the focal length of the Fresnel lens changes the size of the beam spot on the photosensitive drum, which degrades the picture quality.